1. Field of the Invention
The present invention relates to rechargeable lithium batteries, and more particularly to a fabrication method for lithium batteries with self-adhesive polymer electrolyte.
2. Description of the Prior Art
In recent years, rechargeable batteries have been widely recognized for their performance by the manufacturers of portable electronic devices. As a result, various specifications and numbers for different applications have expanded rapidly. Moreover, as the design of electronics devices, information and communication devices have all focused on getting smaller and lighter in size, the performance and characteristics of the rechargeable batteries powering them have become an important factor in various products"" features.
FIG. 1 and FIG. 2 are schematic representations of the conventional fabrication method for cathode/anode plates of rechargeable batteries. Firstly, cathode/anode active substances and polymer binder, conductivity promoter are mixed to form cathode/anode materials 12a, 12b. Next, the cathode/anode materials are coated onto electron collectors 11a, 11b using a coating machine. FIG. 1 and FIG. 2 differ in the way the coating is cast, with discontinuous coating in FIG. 1, and continuous coating in FIG. 2. The coated cathode/anode plates 10a and 10b are then pressed and formed into strips. Polymer separators are used to isolate the cathode/anode plates from each other. The cathode/anode plates are then rolled to form a cylindrical or oval shape, followed by placement in a cylinder or rectangular metal shell 21. Liquid electrolyte is then poured into the metal shell. The shell is then sealed to complete the fabrication of a cylindrical or rectangular battery.
Recently, with the rapid development of electronic devices, the improvement of rechargeable batteries has focused on the need for smaller sizes, lighter weights, and high energy density. The structure of a traditional rechargeable battery is shown in FIGS. 3 and 4, where rolled cathode plate/polymer separator/anode plates 20, 20a are formed into a circular or rectangular shape and placed in a circular metal shell 21, 21a. This circular structure is quite mature in the industry. However, the metal shell will increase the weight of the battery. Therefore, future trends make use of aluminum foil to package batteries, reducing weight.
Consequently, packaging technology for batteries consists primarily of stacking or rolling, i.e. the stacking method of the so-called current polymer battery (Bell Laboratory, US), as shown in FIG. 5. Theoretically, stacked plates 30 is the most densely packed mode, that is, the space is used the most efficiently. Moreover, with the aluminum shell 31, the weight of the battery is reduced. Consequently, the energy density of the battery is improved as well. However, the non-adhesive nature of the plates and polymer separator (PE, PP or nonwoven fabric) has kept the stacking method from being widely applied in rechargeable batteries (Ni-MH battery, Li-ion battery).
Polymer batteries can be stacked tightly together because the polymer electrolyte film (similar to separators) is more adhesive. However, plates need to be stacked tightly with polylmer electrolyte (films), and therefore, a polymer adhesion agent must be used on cathode/anode plates. The adhesion agent, however, reduces the proportion of active substances on cathode/anode plates, and consequently the energy density is less than that of Li-ion batteries. Furthermore, the adhesion agent causes the reduction of the conductivity of cathode/anode plates, which induces problems in recharging/discharging batteries. Therefore, it is necessary to develop a method for fabricating batteries with sufficient adhesion between plates and separators, while maintaining the energy density and conductivity without the addition of polymer adhesion agents.
There have been proposals using adhesive substances on plates to solve the above problems, however, the manufacturing process for stacked polymer batteries is not the same with the current fabrication process for Lithium batteries. In addition, production and technique for massive production of stacked polymer batteries are not developed enough to be widely adopted. Hence, a novel fabrication method compatible with the current fabrication process is in demand.
The object of the present invention is to provide a method for fabricating a lithium battery with self-adhesive polymer electrolyte. The method is not only close to the present process for fabricating lithium batteries, but also applies to stacked polymer batteries.
Another object of the present invention is to provide a method for fabricating polymer batteries that is compatible with the present manufacturing process for batteries.
Still another object of the present invention is to provide a battery with adhesive polymer electrolyte and a battery able to provide high energy density.
To achieve the above-mentioned objects, a battery with electrode plates and separators is firstly filled with a polyacrylonitrile-based solution, followed by the addition of an organic solvent to phase-separate the polyacrylonitrile solution to bond the electrode plates and separators together.
The characteristic step of the invention is the heating and dissolving of polyacrylonitrile in a solution, and pouring the polyacrylonitrile-containing solution into a battery while the solution is still in a molten state and flows as a liquid. The means for filling the battery is not limited to pouring only, injecting and dispensing are also applicable. The battery is not limited to rolled plate structure, stacked plates can be applied as well. After the solution is cooled, an organic solution is added to phase-separate the gellacous polyacrylonitrile. Polyacrylonitrile will be separated to form polymer films or bulks, thus bonding the plates and separators together in the battery. This adhesion procedure is compatible with the current manufacturing technique for lithium batteries.
The solution used in the invention is preferably ethylene carbonate, propylene carbonate, or a combination thereof.
A suitable organic solvent is selected from diethyl carbonate(DEC), dimethyl carbonate(DMC), ethyl methyl carbonate(EMC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), r-butyrolactone, 1,2-dimethoxyethane (DME), tetrahydrofuran (THF), 2-methyltetrahydrofuran (2MeTHF) and vinylene carbonate(VC) etc. The above compound can be used alone or as a combination of more than two solvents.
A large number of ions are needed for the electrochemical reaction involved in a battery, and thus, either the solution or the organic solvent contains lithium salts, or they can both contain lithium salts, preferably LiPF6, LiClO4, or LiBF4. The preferred concentration of the lithium salts is 0.5-3M.
According to this invention, the adhesion between plates and separators is excellent, meanwhile, the required ion concentration is also achieved, which means the batteries made has high energy density.